Various different methods have been developed for separating gases and producing a concentrated stream of a selected gas. One particular method which has been used in industry is the pressure swing adsorption process. Generally, these processes use an adsorbent which, under elevated pressure conditions, preferentially adsorbs a targeted gas over other gases present in a gas stream. Accordingly, the adsorbent could be selected to preferentially adsorb an undesirable gas from a gas stream thereby leaving a gas stream having an increased concentration of the gases remaining in the gas stream. An example of such a process would be the use of a pressure swing adsorption process to produce an oxygen enriched air stream. The adsorbent would be selected to preferentially adsorb nitrogen over oxygen. Thus, after the adsorption process is conducted, the pressurized air in contact with the adsorbent contains a higher percentage by volume of oxygen. This oxygen enriched air may then be vented from the adsorption zone and the adsorbent purged (at reduced pressure conditions) to remove the adsorbed nitrogen. Alternately, such a process may be used to preferentially adsorb a targeted gas (e.g. oxygen) thereby also producing an enriched stream of oxygen.
Various different processes have been designed to utilize the selective adsorption ability of zeolite. Examples of these include, Bansal (U.S. Pat. No. 4,973,339), Stanford (U.S. Pat. No. 4,869,733) and Haruna et al (U.S. Pat. No. 4,661,125).
The process and apparatus of Bansal, Stanford and Haruna et al each utilize two adsorption zones. The use of two adsorption zones is undesirable as it unnecessarily complicates the apparatus since it requires additional valving and control means to cycle each adsorption bed through a pressurization cycle and a purging cycle. Further, this adds to the cost of the apparatus and decreases the reliability of the apparatus.
Other disadvantages of existing designs is the requirement to use expensive valve control means. In particular, solenoids are frequently required to switch the adsorption zone from a pressurization mode to a purging mode. These controls are expensive and also prone to failure after extensive use.
Further, existing designs utilize electronics (e.g. microprocessors) to control the cycling of the adsorption zone. This adds to the cost of the equipment and also requires an electrical power source to operate the process. Further, the electronic components may be damaged in harsh environments and this limits the applications of some existing designs.